The conventional way of fertilizing water plants is to use a timed released plug that is inserted into the soil once every two to three weeks. The plug releases chemical fertilizer into the soil which is absorbed by the plant's root system.
Another product that allows a water plant to absorb much needed nutrients is a design that works as a filtration device and a fertilizer. Waste is pulled through a bag containing material to trap fish waste, bacteria, and suspended particles. The water lily, a tuberous root plant, is placed in the middle of this bag and is supposed to absorb the nutrients as they are collected.
However, one of the concerns expressed by individuals involved with water gardening, is that many people wish to garden organically. There is currently no product or system that allows an organic gardener to fertilize their aquatic plants.